This invention relates to plasma torches used for spectrochemical analysis. In particular, this invention relates to a plasma torch designed to analyze samples of air in an inductively coupled plasma.
Plasma torches, since their inception in the early 1970s, have been designed expressly for use with plasma gases of homogeneous composition. This is in contrast to the invention to be described hereinbelow which involves the introduction of sample air into argon plasma. Consequently, the design of these prior art torches has been optimized for homogeneous gases, primarily, because the prior art has not recognized the need to introduce air into argon plasma, see, for example, U.S. Reissue Pat. RE 29,304. Since argon inductively coupled plasmas are used exclusively for the elemental analysis of liquid samples such as water, and maximum sensitivity is often required, it has been found that the addition of other gases typically degrades analytical performance. Consequently, the introduction of air into argon plasmas has been avoided and the absence of this requirement until now has left the problems associated with air introduction unsolved.
Referring to FIG. 1, typical prior art plasma torch T consists of three concentric quartz tubes that are fused or otherwise held together using appropriate hardware. These tubes are commonly called outer tube OT, tulip-shaped intermediate tube IMT and central injector tube CIJT. Three distinct argon streams flow through these tubes. Typically, through narrow annulus NA formed between outer tube OT and tulip-shaped intermediate tube IMT, plasma argon PA flows at rates of 15-20 liters per minute. In the large annulus LA located between intermediate tube IMT and central injector tube CIJT, auxiliary argon AXA typically flows at 0-2 liters per minute. Carrier or aerosol argon CA flows at 0.5-1 liter per minute through central injector tube CIJT.
The narrow annulus NA in this classic torch geometry that was formed between tulip-shaped intermediate tube IMT and outer tube OT adequately provided a suitable flow of plasma gas PA. However, the velocity of auxiliary argon AXA through the area inside of the "tulip" has been found to be insufficient to promote satisfactory injection of sample air with carrier aerosol CA into plasma fireball PF and produce accurate results or avoid damaging the tubes. The state-of-the-art torches have had great difficulty when air flow of approximately 0.5 liters per minute is added to the flow of argon carrier CA through carrier injector tube CIJT since some portion CAa of carrier CA would inadvertently flow around the fireball. The geometry of standard plasma torches makes air injection difficult to achieve under analytically-favorable conditions, and failure to achieve air injection leads to accelerated aging of these torches.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for plasma torches that provide reliable analyses of airborne samples.